The present invention relates generally to methods and devices for extending the operating life of fixed bed reactors.
In the operation of fixed bed reactors, the top of the bed often becomes fouled or plugged by the deposition of fouling materials (also referred to as particulates, particulate impurities, or foulants). Examples of fouling materials include organometallic compounds, polymeric materials, carbonaceous materials, organic particulates and inorganic particulates. The plugging of the bed results in increased pressure drop that may necessitate shutdowns, throughput reduction, and time consuming repairs and maintenance.
To overcome this problem, many methods have been devised that require equipping each reactor with more than one bed and bypassing a fouled bed. Examples of such methods are described in U.S. Pat. Nos. 3,509,043 and 4,313,908. One shortcoming of such methods is that they require an auxiliary bypassable bed. Thus, the above methods do not readily apply to single fixed bed reactors.
Other methods involve the use of trash baskets. For example, U.S. Pat. Nos. 3,992,282 and 3,888,633, describe a fixed catalyst bed reactor having a hollow trash basket made from a screen mesh material (or scale trap) that extends into the catalyst bed. Particulate impurities are removed from a fluid stream flowing into the fixed catalyst bed by the hollow trash basket.
While the trash baskets tend to remove fouling materials contained in the fluid feedstocks, they generally have only a small effect in minimizing pressure drop buildup due to fouling. This is partially because fouling materials plug the trash basket walls within a short period of time. Thus, the flow passage of the fluid feedstocks is occluded and the pressure drop begins to rise, though at a somewhat slower rate than if the trash baskets were not used. Generally, it is desirable to keep fixed bed reactors on stream without significant pressure drop buildup for a long time, very often for several years. Thus, the methods involving trash baskets do not provide adequate protection against pressure drop buildup, and other methods are needed to further extend the operating life of fixed bed reactors. Other problems are associated with existing methods for extending the operating life of fixed bed reactors, contactors and filters.
One aspect of the present invention relates to a multi-tier bypass device that can be readily added to a fixed bed reactor for extending the operating life of the reactor. The multi-tier bypasss device comprises a cage having a plurality of successive chambers that are in fluid communication with one another. Each chamber may have a plurality of perforations for allowing any bypass flow that enters the chamber to exit the chamber and enter a clean bed layer surrounding the chamber. Each chamber, except the last chamber in the cage, may also have a fluid communication device for allowing any bypass flow that enters a chamber that is surrounded with a fouled layer to pass into the next chamber. This process is repeated until the bypass flow enters the last chamber and exits from that chamber into the last unfouled layer of the bed through side and/or bottom perforations.
The multi-tier bypass device also may include a bypass tube in fluid communication with at least one chamber of the cage. The bypass tube may protrude from the cage above the fixed bed for bypassing an increasing amount of the feedstock around a fouled layer of the fixed bed. The bypass flow will pass through the bypass tube into a chamber of the cage and out from that chamber through the chamber perforations into a clean bed layer. The multi-tier bypass device effectively partitions a single bed in multiple layers corresponding generally to the number of chambers in the cage.
The present invention also relates to an improved fixed bed reactor comprising a fixed bed for transforming a feedstock flowing therethrough, and at least one multi-tier bypass device positioned in the fixed bed for allowing the feedstock flow to bypass successive layers of the fixed catalyst bed as they foul.
Yet another aspect of the invention is directed to a method for extending the operating life of a fixed bed reactor. The method comprises partitioning the fixed bed into a plurality of successive layers, introducing a hydrocarbon feedstock into the fixed bed and as each successive layer fouls, bypassing an increasing amount of the feedstock to the next layer of the fixed bed that is not fouled.
The invention will become better understood with reference to the following detailed description considered in conjunction with the accompanying drawings described below.